1. Field of the Invention
The present invention relates to a driving device which changes the orientation angle of optical components in laser apparatus.
2. Description of the Related Art
When an excimer laser or molecular fluorine F2 laser is used as a stepper light source, the oscillated laser light must be subject to line-narrowing. Also, in order to prevent the central wavelength of the spectrum of the oscillated laser light with narrowed linewidth from being deviated from the target value during exposure, stability control must be performed precisely.
FIG. 4(a) shows a construction surrounding a line-narrowing unit 9 mounted on a conventional laser apparatus, Alternate long and short dash lines in FIG. 4(a) show optical axes of laser light L. FIG. 4(b) is a cross-sectional view taken along line IVxe2x80x94IV in FIG. 4(a).
The linewidth narrowing effect is achieved by reflecting the laser light at a certain fixed angle of incidence (angle of reflection) xcfx86 relative to a reflector-type wavelength selecting element, or grating 2, installed inside the line-narrowing unit 9. The reflector-type wavelength selecting element 2 is an optical component that selects a wavelength by reflecting the laser light L. Reflector-type wavelength selecting element 2 select a wavelength that corresponds to the angle of incidence (angle of reflection) xcfx86. The angle of incidence xcfx86 is determined by the orientation angle xcex8 of the reflection mirror 3 which totally reflects the incident laser light L and directs it unto the reflector-type wavelength selecting element 2. The orientation angle xcex8 of reflection mirror 3 is changed by linear movement of a feed screw 16a (male screw) of the feed screw unit 16 in the directions of arrows B. With rotational movement of a control motor 8 as shown by arrow C, the feed screw 16a moves linearly.
During exposure the control motor 8 is drive controlled so as to make the deviation of the central wavelength of the oscillated laser spectrum to be extremely small. Specifically, clockwise or anticlockwise rotation of the drive shaft 8a of the control motor 8, as shown by arrows C, makes the feed screw 16a to rotate in the same direction C as the drive shaft 8a. Rotation of the feed screw 16a results in its reciprocal movement in the direction of arrow B, thus determining the feed position of the feed screw 16a. This, in turn, determines the orientation angle xcex8 of the reflection mirror 3 and the angle of incidence xcfx86 of the light relative to the reflector-type wavelength selecting element 2, ad the central wavelength of the laser light L spectrum is thereby fixed at the target wavelength.
Here, a so-called high-precision screw is used for the feed screw unit 16. The high-precision feed screw is machined so as to make the clearances between the feed screw 16a (male screw) and the nut 16b (female screw) to be extremely small. Specifically, each pair of feed screw 16b and nut 16b is managed by spot goods control. To reduce the contact friction between the feed screw 16a and the nut 16b, a lubricant (grease) is applied in abundant quantity onto a part D where the feed screw 16a and the nut 16b are contact with each other.
However, even if the thread clearances in the feed screw unit 16 can be made very small by spot goods control, it is structurally impossible to make them extremely small, and very small thread clearances inevitably exist In other words, the backlash in the feed screw unit 16 cannot be completely eliminated. Therefore, when the control motor 8 rotates back and forth in the direction of arrows B, and the feed screw 16a moves reciprocally in the directions of arrows B, precision of positioning the reflection mirror 3 is impaired, and if the precision of positioning the reflection mirror 3 is impaired, precision of controlling wavelength of laser light T, to be a target value is impaired, resulting in impaired wavelength stability. In other words, there is a problem of decreased precision in controlling wavelength and decreased wavelength stability.
Another drawback of the conventional feed screw unit 16 is that due to very small thread clearances, significant friction resistance exists during operation of the feed screw unit 16. This leads to increased rotation torque of control motor 8 that is necessary for driving the feed screw unit 16. This results in poor linear movement response of the feed screw 16a to the drive command to the control motor 8, and a considerable time is required to change the wavelength of the laser light 1, to the target wavelength. Therefore, there is a problem of decreased speed of wavelength control.
In order to decrease the friction resistance during operation of the feed screw unit, it is necessary to apply an abundant quantity of a lubricant on contact section T). However, contact section D communicates with the interior of the line-narrowing unit 9. The excimer laser light L of deep ultraviolet wavelength zone scatters within the line-narrowing unit 9. This deep ultraviolet light irradiates the lubricant and may promote the chemical decomposition reaction of the lubricant. If the decomposition reaction of the lubricant proceeds, the impurities thereby produced will pollute optical components located inside the line-narrowing unit 9, and the performance of the laser apparatus may be impaired. Therefore, there is a problem of poor performance of the laser apparatus resulting from pollution of optical components.
With the foregoing in view, it is an object of the present invention to improve precision of wavelength control and wavelength stability, to increase speed of wavelength control and to prevent optical components from being polluted.
To achieve this object, the present invention provides an optical component driving device provided in a laser apparatus, comprising an optical component (3) that changes wavelength of the laser light (L) in accordance with an orientation angle (xcex8)thereof, and a feed screw mechanism (6) that convert rotational movement of a rotary actuator (8) into linear movement of a feed screw (6a), the orientation angle (xcex8) of the optical component (3) being changed in accordance with the near movement of the feed screw (6a) of the feed screw mechanism (6), characterized in that the feed screw (6a) of the feed screw mechanism (6) is a ball screw (6a).
The present invention will be described with reference to FIGS. 1(a) and 1(b).
The ball screw unit 6 is a feed screw mechanism that is used by previously applying a load (pre-load) between a ball screw 6a and a nut 6b and in which the ball screw 6a moves linearly by the sliding of the ball. This construction permits to make the mechanical screw clearances to be very small, thus making it possible to eliminate the backlash of the ball screw unit 6, or to make it non-backlash. Since there is no screw clearance, it is possible to improve the precision of positioning the reflection mirror 3 when the control motor 8 rotates clockwise and anticlockwise as shown by arrows C, and the screw 16a moves reciprocally a shown by the arrows B. Better precision of positioning of the reflection mirror 3 results in improvement in precision of controlling the wavelength of the laser light L to the target wavelength, and improvement in wavelength stability
Since the ball screw 6a is caused to move linearly by the sliding of the balls, friction resistance during operation of the ball screw unit 6 is low. Therefore, smaller torque of the control motor 8 is required to drive the ball screw unit 6. As a result, linear movement response of the ball screw 6a to the drive command to the control motor 8 can be increased, while the time required for adjustment of the wavelength of the laser light T, to the target value can be reduced.
Furthermore, due to better wavelength control precision and reduced time of adjustment of light wavelength to the target value, the speed of wavelength control can be increased.
Due to low friction resistance of the ball screw unit 6, the amount of lubricant applied to contact portion D can be minimized. As a result, in spite of deep ultraviolet light irradiating the lubricant, the amount of resulting impurities is negligible. This makes it possible to minimize pollution of optical components located inside the line-narrowing unit 9 and to prevent deterioration of the laser apparatus performance.